Traditionally, as they are manufactured, solar cells are tested for their current at a given voltage across the cell, a given level of solar radiation incident upon the face of the cell and a given temperature. Then, they are separated according to their tested currents.
Typically, in forming arrays from such manufactured solar cells, e.g., during the process of constructing solar panels, solar cells which have been separated into one of a number of current ranges are then included in a particular panel.
By way of example, each cell of the yield of P/N junction solar cells from a conventional manufacturing process might be tested at a voltage near the expected maximum power point for such cells, at a level of solar radiation incident on the cell face of 1 kilowatt/(meter).sup.2 (1 sun of insolation) and at a temperature for the solar cell environment, and thus, for the cell, of 28 degrees centigrade (approximating room temperature). The maximum power point for a cell at a given level of incident solar radiation and temperature, of course, is the point of maximum voltage multiplied by current along the voltage versus current curve for that cell under those conditions.
Upon such a testing of a large group of solar cells manufactured at about the same time, with certain highly atypical (though perhaps excellent) and extremely poor cells being subsequently eliminated, a distribution of currents, tested in the fashion indicated, for the remaining group, is expected. The distribution, for example, might cover a range of about one ampere. An average test current for such a remaining group of solar cells can readily be calculated; and experience shows that the average for one such large group can generally be estimated as the average for a prior such large group manufactured at the same plant, near the same time, by substantially the same process.
As an example, for a manufacturing process yielding five to ten thousand solar cells a day, one might typically expect the average for any large number of cells as they are manufactured, with certain cells eliminated as indicated, to hover in the vicinity of 2.1 amperes for a number of months, but during another period of months to hover, e.g., at 1.9 amperes. This, of course, is just by way of illustration.
Returning to the forming of arrays of such solar cells, for example in constructing solar panels, for a particular array or panel design, solar cells having a test current within a range such as 0.1 ampere would conventionally be selected for the particular design. A fundamental reason for this matching is to avoid the negative effects which can occur when cells with significantly varying characteristics are employed in a single array. Concerning solar cells connected in series, the connection of solar cells having identical characteristics is the ideal situation. Then, under ideal conditions, the cells will all operate at their identical maximum power points with, as is required by the series connection, the same current passing through each cell. On the other hand, if a cell having a significantly lower voltage along its characteristic, than the other cells, at the maximum power point current for the other cells, is included in the series, the other cells will force a current through the differing cell that is significantly higher than its maximum power point current, thus moving the operating point of the differing cell to a significantly lower voltage than at its maximum power point. In an extreme case, the voltage could become a voltage with a different polarity than for the other cells. This situation causes a power output for the total series connection that is below the ideal.
The present invention provides a fabrication process for solar to electrical energy conversion units which permits the effective utilization of a wide distribution of solar cells while, at the same time, fabricating units having a high degree of uniformity of operating characteristics. It accomplishes this through the categorization of the cells and the formation of arrays of the cells which take rather specific, but varying forms.